Electrochemical grinding tool and method

ABSTRACT

An electrochemical grinding tool and method capable of rounding sharp edges that may be prone to cracking, for example, edge regions of cooling slots within dovetail slots of turbine wheels. The electrochemical grinding tool includes a drilling assembly, a conductive bit, and a motor for rotating the conductive bit about an axis thereof. The conductive bit of the electrochemical grinding tool is inserted into a first slot, an electrolyte solution is applied between the conductive bit of the electrochemical grinding tool and a second slot that intersects the first slot, an electrical potential is applied to the conductive bit and the turbine wheel to create a potential gradient between the conductive bit and the edge of the second slot, and material is removed from the edge of the second slot by displacing the conductive bit about and along the edge.

BACKGROUND OF THE INVENTION

The present invention generally relates to grinding tools and methods.More particularly, this invention relates to methods and systems formachining sharp edges of a slot that can be prone to cracking, forexample, edge regions of slots within turbine wheels employed inturbomachines, including but not limited to gas turbines used in powergeneration.

In the hostile operating environments of gas turbine engines, thestructural integrity of turbine rotor wheels, buckets, and othercomponents within their turbine sections is of great importance in viewof the high mechanical stresses that the components must be able tocontinuously withstand at high temperatures. For example, the regions ofa turbine wheel forming slots into which the buckets are secured,typically in the form of what are known as dovetail slots, are known toeventually form cracks over time, necessitating monitoring of the wheelin these regions. In some wheel designs, nonlimiting examples of whichinclude the stage 1, 2, and 3 wheels of the General Electric 9FB gasturbine, cooling of the buckets and wheel perimeter is assisted by thepresence of a cooling slot located near the perimeter of the wheel andinto which the dovetail slots extend. Over extended periods of timeunder the severe operating conditions of a wheel, cracks may form atcommon edges formed where the dovetail slots and cooling slot intersect.Optimization of the cooling slot geometry to reduce the likelihood ofsuch cracks is desirable in order to improve expected life of a turbinewheel.

While a turbine rotor can be completely disassembled to gain access toits individual wheels, grinding techniques that can be performed withlimited disassembly are preferred to minimize downtime, such as to fitwithin outage schedules of a gas turbine employed in the powergenerating industry. However, access to the cooling slot is verylimited, and any grinding technique must address the difficulty ofbringing the tool into stable proximity to the edges being rounded.

Currently, cooling slots of gas turbine engines are generally rounded bymechanical grinding followed by a finishing process, such as BPP (blend,polish, peen). These methods involve using a bit to remove material atthe edge of the cooling slot and then blending and/or polishing theedges to obtain the desired radius of the intersection edges. However, adesired radius is often difficult to achieve if the grinding waspreformed by mechanical means. Furthermore, BPP methods may fail toremove all of the cracks in the cooling slots.

Therefore, it would be desirable if a method existed by which sharpedges prone to cracks on a turbine wheel, particularly edge regions ofslots within the wheel, could be rounded to a desired radius withminimal polishing and/or blending. It would also be desirable if such aprocess were able to be performed without necessitating completedisassembly of a turbine rotor to gain access to its individual wheels.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides electrochemical grinding tools andmethods capable of rounding sharp edges that may be prone to cracking,for example, edge regions of cooling slots within a dovetail slot of aturbine wheel.

According to a first aspect of the invention, a method is provided forrounding an edge of a first slot that intersects at least a second slotof a component. The method entails the use of an electrochemicalgrinding tool comprising a drilling assembly, a conductive bit, meansfor rotating the conductive bit about an axis thereof, and means forapplying an electrical potential to the conductive bit. The conductivebit of the electrochemical grinding tool is inserted into the secondslot of the component, an electrolyte solution is applied between theconductive bit of the electrochemical grinding tool and the first slot,an electrical potential is applied to the conductive bit and thecomponent to create a potential gradient between the conductive bit andthe edge of the first slot, and material is removed from the edge of thefirst slot by displacing the conductive bit about and along the edge.

According to a second aspect of the invention, an electrochemicalgrinding tool is provided that is adapted to round an edge of a firstslot within at least a second slot of a component. The electrochemicalgrinding tool includes a drilling assembly, a conductive bit rotatablymounted to the drilling assembly, means for rotating the conductive bitabout an axis thereof, means for applying an electric potential to theconductive bit, and means for securing the electrochemical grinding toolto at least the second slot of component while performing anelectrochemical grinding operation on the first slot of the component.

A technical effect of the invention is the ability to mount a grindingtool directly to a component, for example, a turbine wheel, for roundingedges of the component that may be prone to cracks. The use of theelectrochemical grinding tool is particularly advantageous for roundingedge regions of cooling slots of turbine wheels to achieve a desiredradius with minimal polishing and/or blending. Another advantage of theinvention is the ability to employ the grinding tool withoutnecessitating complete disassembly of a turbine rotor to gain access toits individual wheels.

Other aspects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 represents a fragmentary perspective view showing a cooling slotand two dovetail slots of a turbine wheel.

FIG. 2 represents a fragmentary perspective view of a turbine wheel andan electrochemical grinding tool engaged therewith in accordance with anembodiment of the invention.

FIG. 3 represents a cross-sectional view showing the outer axial edge ofthe turbine wheel of FIG. 2 and the electrochemical grinding toolengaged therewith.

FIG. 4 represents a cross-sectional view showing the cooling slot ofFIG. 3 after completion of the rounding process.

FIG. 5 represents a perspective view showing the electrochemicalgrinding tool of FIG. 2 disassembled from a support assembly and asuction assembly.

FIG. 6 represents a side view showing the electrochemical grinding toolof FIG. 5 with a protective cover removed.

FIG. 7 represents a cross-sectional view taken along section line 6-6 ofFIG. 6.

FIG. 8 represents a top view of a support plate of the electrochemicalgrinding tool of FIG. 6.

FIG. 9 represents a bottom view of the electrochemical grinding tool ofFIG. 6.

FIG. 10 represents a plan view of a drilling assembly of theelectrochemical grinding tool of FIG. 6.

FIGS. 11 and 12 represent cross-sectional views of the drilling assemblyof FIG. 10 taken along section lines 10-10 and 11-11, respectively.

FIG. 13 represents an exploded perspective view of a suction assembly ofthe electrochemical grinding tool of FIG. 2.

FIG. 14 represents an exploded perspective view of a support assembly ofthe electrochemical grinding tool of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in terms of methods and anapparatus for machining an edge region of an article, for example, torepair and optimize the geometry of high stress edge regions of anarticle that are prone to cracking While various applications areforeseeable and possible, applications of particular interest includedifficult to access regions of components of gas turbines, includingland-based gas turbine engines. Of more particular interest are turbinewheels having axial dovetail slots along a perimeter thereof that areconfigured for mating with and securing airfoil members to the perimeterof the wheel, and an annular cooling slot that intersects the axialdovetail slots. A fragmentary view of such a turbine wheel 10 isrepresented in FIG. 1 and will serve as an example in the followingdiscussion.

FIG. 1 depicts two dovetail slots 14 of the turbine wheel 10, which isrepresentative of the type conventionally used in gas turbine enginessuch as those used in the power generation industry. An annular coolingslot 12 intersects the axial dovetail slots 14. The cooling slot 12comprises side edges 16 and radially-outward edges 18. If these edges 16and 18 are sufficiently sharp, cracking can occur in regions of thecooling slot 12. As an example, cracking has been observed to occur nearthe intersection of the aft side edge 16 and the radially-outward edge18 looking down stream of a gas turbine engine. Removing the turbinewheel 10 from the machine for the purpose of repairing or optimizing thegeometries of these edges 16 and 18 is a long-lead, high-cost operation.The method and apparatus herein described provides a means of repairingand optimizing the geometry of the turbine wheel 10 in-situ in thecase-off condition to reduce stress concentrations, for example,attributable to the geometries of the cooling slot edges 16 and 18.According to a preferred aspect of the invention, the method andapparatus entail an electrochemical chemical grinding (ECG) process thatis capable of repairing the edges 16 and 18 by removing any damagedmaterial and simultaneously rounding mating surfaces that form the edges16 and 18. Peening may be used in a follow-on operation to apply asurface compression layer.

ECG is a low-force machining operation where electrochemical oxidationand light abrasive machining dominate the material removal process.Machined feature edges are naturally broken or created with radii. ECGprocesses can use specific tool electrodes to machine and generatesurface features. Tool electrode materials are often copper, aluminumoxide, and a resin bonding material that cements the copper and ceramictogether. ECG processes also use a conductive abrasive tool to machinefeatures in parts. A power supply is connected to the conductiveabrasive tool and a part to be machined to drive a potential gradientbetween the tool and part. This potential gradient is used to adjust thematerial removal rate and balance between anodic dissolution andabrasive grinding. The energy field intensity generated by the potentialgradient can be adjusted by changing the applied potential and the toolposition to consistently round the edges of a machined region. Anelectrolyte is typically flushed between the tool and part to removemachining swarf, chips, and dissolved metal ions. The removal ofmaterial and rounding of edges can be achieved in a single machiningoperation. Corners and edges typically have high field gradients, withthe result that material removal rates at edges are normally greaterthan at flat surface regions, such that round corners are a naturalartifact of ECG.

FIG. 2 depicts an ECG tool 24 secured to the turbine wheel 10 of FIG. 1in accordance with an embodiment of the present invention. According toa preferred aspect of the invention, the ECG tool 24 includes a supportassembly 22 adapted to mount the tool 24 to at least one dovetail slot14 of the turbine wheel 10. The support assembly 22 (shown in moredetail in FIG. 13) is mounted to the ECG tool 24, for example, withbolts or some other suitable means. In FIG. 3, dovetail locators 96 ofthe support assembly 22 are shown as being individually engaged with twodovetail slots 14 located on either side of an intermediate slot 14. Asalso represented in FIG. 3, the ECG tool 24 is adapted to lower aconductive bit 38 into a position near the edges 16 and 18 of a coolingslot 12 within the intermediate slot 14 of the wheel 10. Once theconductive bit 38 is in position, a potential gradient is preferablyapplied between the conductive bit 38 and a surface to be machined withthe tool 24. The conductive bit 38 is rotated about its axis to removematerial and round the edges 16 and 18 of the cooling slot 12 within theintermediate slot 14. FIG. 4 represents edges 16 and 18 of cooling slot12 that have been rounded in accordance with a preferred aspect of thisembodiment. As represented in FIG. 4, a corner formed by the edges 16and 18 has been removed and replaced with a depression recessed in thecomponent and the depression is surrounded with rounded edges.

FIG. 5 represents the ECG tool 24 with the support assembly 22 removedtherefrom. Handles 27 are located on sides of a support plate 30 and ahoist ring 28 is located on the outermost surface of the support plate30. A protective cover 20 surrounds components of the ECG tool 24,including a drilling assembly 36 (FIGS. 6 and 10), a servomotor 32, anda motor 34. The support assembly 22, servomotor 32, motor 34, anddrilling assembly 36 are all mounted to the support plate 30 so that thetool 24 can be installed and removed from the turbine wheel 10 as aunitary assembly.

FIG. 6 represents the ECG tool 24 with the protective cover 20 removedto expose the drill assembly 36 mounted on the support plate 30. Theconductive bit 38 is mounted to the drill assembly 36 with a conductivespindle 40 that protrudes from the drill assembly 36. While not limitedto any particular type of bit, the conductive bit 38 may be, forexample, a 0.5 inch (1.3 cm) bit of a type commercially available. Asrepresented in FIG. 3, the bit 38 is preferably held at a predeterminedangle to edges 16 and 18 that corresponds to the design criteria forproducing the desired material removal to reduce high stress areas ofthe cooling slot 12. As represented in FIG. 4, the predetermined angleis not parallel to, perpendicular to, or in-plane with any intersectingsurfaces of the cooling slot 12 and the dovetail slot 14 orperpendicular to the edges 16 and 18. Similarly, according to theembodiments represented in FIGS. 1 and 4, the predetermined angle is notparallel to radials or an axis of rotation of the turbine wheel 10. As anonlimiting example, the intersecting planes of the dovetail slot 14 arepreferably desired to have a radius of about 0.030 to about 0.090 mils(about 0.76 to about 2.3 micrometers) or larger as long as the radiusdoes not create any visible edges along the cooling slot 12. Theconductive bit 38 plunge speed can be determined by the amount ofpressure applied to the ECG tool 24 by the operator, a servo, pneumaticpiston system, hydraulic system, or any other suitable means or methodcapable of delivering pressure to the tool 24. Acceptable feed rates arebelieved to be about 0.01 to about 1 inch (about 0.25 to 25 mm) perminute. A nonlimiting example of an acceptable plunge distance forachieving a desirable geometry in the cooling slot 12 is approximately0.125 inches (about 0.3 centimeter).

In the embodiments represented in the figures, the plunge speed andposition of the conductive bit 38 are controlled by a servomotor 32. Inparticular, the servomotor 32 is coupled with a ball screw 60 to a ballnut housing 62 to which the drill assembly 36 is mounted. The servomotor32 can be paired with an encoder (not shown) to provide position andspeed feedback to determine the plunge speed, thereby eliminating theneed for operator intervention during the machining operation. Theservomotor 32 can be mounted to the support plate 30 in a manner asrepresented in FIGS. 6 and 7. As more readily evident from FIG. 8, acoupling 58 connects the ball screw 60 and ball nut housing 62 to theservomotor 32. As the servomotor 32 displaces the drill assembly 36, thedrill assembly 36 is translated on linear slide assemblies 64. The drillassembly 36 is mounted to carriages 66 of the slide assemblies 64 byscrews 42, as represented in FIG. 7. Referring again to FIG. 8, thecarriages 66 slide on rails 68 along a longitudinal axis of the ECG tool24. Travel stops 70 are located near the ends of the rails 68 to retainthe carriages 66 on the rails 68 and to limit the distance the carriages66 may travel along the rails 68.

FIG. 10 represents the drilling assembly 36 disassembled from thesupport plate 30. The drilling assembly 36 is represented as comprisingbrush housings 76, a motor flange 80, and a spindle housing 74. Asrepresented in FIG. 11, the brush housings 76 secure brush assemblies 78and collectors 48 that serve to complete an electrical circuit between afixed conductor (not shown) and the rotating spindle 40 and bit 38. Awire 44, shown in FIG. 9, may be connected to an external power source(not shown) which supplies electricity to the brush assemblies 78. Asrepresented in FIGS. 6 and 9, a lug ring 46 connects the wire 44 to thebrush assemblies 78 and a clamp 72 connects the wire 44 to the outermostsurface of the support plate 30. The external power source, brushassemblies 78 and collectors 48 provide the means by which the potentialgradient may be applied between the cooling slot 12 and the spindle 40.Suitable potential gradients are believed to be over a range of about 2to about 20 volts, though the use of lower and higher potentialgradients is also foreseeable.

The drilling assembly 36 is represented in FIG. 10 as further comprisinga motor mounting flange, 82 that secures the motor 34 to the motorflange 80. The motor 34 provides the means by which the conductive bit38 is rotated on its axis. Suitable rotational speeds for the conductivebit 38 are believed to be about 500 to about 40,000 RPM, preferablyabout 20,000 RPM, though higher and lower speeds are foreseeable. Themotor 34 is preferably an air motor, though it is foreseeable that themotor 34 could be an electric motor, a belt-drive motor, or another typeof motor capable of providing acceptable operational speeds. FIG. 12represents a coupling 88 connecting an axle 92 of the motor 34 to aspindle axle 90 coupled to the spindle 40. The motor 34 rotates the axle92, spindle axle 90, and spindle 40 thereby rotating the conductive bit38. The motor 34 can be connected to a suitable compressed air supply(not shown) with a tube 52. An exhaust muffler 50 is located on themotor 34 for muffling the sound produced by the motor 34 during itsoperation.

The ECG tool 24 is also preferably equipped to flush or mist anelectrolyte solution (not shown) onto surfaces of the cooling slot 12adjacent the edges 16 and 18 and the conductive bit 38. The electrolyteis preferably forced to flow in a manner that does not allow other gasturbine components to be wetted. Preferred electrolytes comprise aqueoussalts, for example, sodium formate, that do not promote pitting orcorrosion of other components of the gas turbine should some electrolyteleak from the flow region. As more readily seen in FIG. 6, a hose 54 isprovided through which an electrolyte solution from a suitable supply(not shown) can be pumped to a modular hose system 56 that directs thesolution at the surfaces intended to be flushed or misted (FIG. 3).

In a preferred aspect of the invention, FIGS. 2 and 3 represent asuction assembly 26 mounted to the support plate 30 of the ECG tool 24.The suction assembly 26 collects the electrolyte solution supplied bythe hose system 56 during the ECG process. The collected electrolytesolution may be re-used or discarded. The solution and suction assembly26 serve to eliminate damaged material removed by the conductive bit 38from the cooling slot 12 as well as eliminate material removed as aresult of rounding the edges 16 and 18 of the slot 12. The suctionassembly 26 provides suction to collect the electrolyte. In theembodiment shown, the electrolyte is collected with the suction assembly26 at the aft end of the dovetail slot 14 being machined, though it isforeseeable that the electrolyte could be collected from otherlocations, for example, the dovetail slots 14 adjacent to the dovetailslot 14 being machined by pulling the electrolyte through the coolingslot 12. An exploded view of a particular example of the suctionassembly 26 is represented in FIG. 13. The suction assembly 26 isrepresented as comprising a manifold 110 that may be connected to asuction source (not shown) by a hose fitting 108. A seal insert and/orvacuum insert 112 that is shaped to closely fit within the dovetail slot14 is used to seal the dovetail slot 14 to be machined during operationto further reduce the likelihood that electrolyte solution will contactsurfaces of the turbine wheel 10 other than those to be machined, asshould be evident from FIG. 3.

As previously noted with reference to FIGS. 2 and 3, the supportassembly 22 secures the support plate 30 to the turbine wheel 10 toposition and stabilize the ECG tool 24. As previously stated, althoughvarious means of supporting the ECG tool 24 are foreseeable, a preferredexample of the support assembly 22 is attached to the turbine wheel 10by interacting with the dovetail slots 14 on either side of the slot 14being machined. FIG. 14 represents a preferred embodiment of the supportassembly 22 as comprising the two dovetail locators 96 that are shapedto engage the dovetail slots 14 of the turbine wheel 10. As evident fromFIG. 3, the dovetail locators 96 are preferably spaced to allow thesupport assembly 22 to be secured to dovetail slots 14 on opposite sidesof the dovetail slot 14 intended to be machined. In FIG. 14, the spacingbetween the locators 96 is maintained by a bracket 106 to which thelocators 96 are mounted. The locators 96 are slidably mounted to thebracket 106 with pins 14, and bolts 100 equipped with springs 98 serveto bias the locators 96 away from the bracket 106 to provide for moresecure engagement between the locators 96 and the slots 14 in which theyare received mounted. Each locator 96 is equipped with a compliant foot94 to reduce the risk of damage to the dovetail slots 14. A wheellocator 102 is attached to the bracket 106 and a stop 104 is connectedto the wheel locator 102 to assist in positioning the support assembly22 at a predetermined position on the turbine wheel 10. During mountingof the support assembly 22 to the turbine wheel 10, the dovetaillocators 96 enter their respective slots 14 through an axial endthereof, and are then slid toward the opposite end the dovetail slots 14until the stop 104 contacts a surface of the turbine wheel 10. The bolts100 can be tightened to increase the clamping pressure between thelocators 96 and the dovetail slots 14 to secure the support assembly 22.

Once the cooling slot 12 has been adequately machined, the cooling slot12 and dovetail slots 14 may be flushed to remove and/or dilute residualelectrolyte solution that may remain. Ultrasonic peening or anotherfollow-on operation may then be used to apply a protective surfacecompression layer.

While the invention has been described in terms of certain embodiments,it is apparent that other forms could be adopted by one skilled in theart. Therefore, the scope of the invention is to be limited only by thefollowing claims.

The invention claimed is:
 1. A method of rounding an edge of a firstslot within at least a second slot of a component at an interfacebetween surfaces of the first slot and the at least second slot, themethod comprising: providing a electrochemical grinding tool comprisinga drilling assembly, a conductive bit, means for rotating the conductivebit about an axis thereof, and means for applying an electricalpotential to the conductive bit; inserting the conductive bit of theelectrochemical grinding tool into the second slot; applying anelectrolyte solution between the first slot and the conductive bit ofthe electrochemical grinding tool; applying the electrical potential tothe conductive bit and the component to create a potential gradientbetween the conductive bit and the edge of the first slot; and removingmaterial from the edge of the first slot by displacing the conductivebit in a direction towards the edge and along an axis of rotation of theconductive bit to remove the edge by creating a depression in thecomponent, the depression being recessed into the component and havingrounded edges.
 2. The method according to claim 1, further comprisingsecuring the electrochemical grinding tool to the component.
 3. Themethod according to claim 1, wherein the edge is a corner formed byintersecting edges at the interface between the first slot and the atleast second slot.
 4. The method according to claim 1, furthercomprising securing a suction assembly to the electrochemical grindingtool, and collecting the electrolyte solution with the suction assemblyduring the removing step.
 5. The method according to claim 1, whereinthe direction that the conductive bit is displaced is at a predeterminedangle relative to the edge and is not parallel or perpendicular with anyof the surfaces of the first and second slots that intersect to form theedge.
 6. The method according to claim 1, wherein the rounded edges havea radius of 0.76 millimeters or larger.
 7. The method according to claim1, wherein the conductive bit is displaced about and along the edge bytranslating the drilling assembly within the electrochemical grindingtool.
 8. The method according to claim 1, wherein the component is aturbine wheel, the first slot is a cooling slot of the turbine wheel,and the second slot is a dovetail slot of the turbine wheel.
 9. Themethod according to claim 8, wherein the direction that the conductivebit is displaced is at a predetermined angle relative to the edge and isnot parallel or perpendicular with radials of the turbine wheel or withan axis of rotation of the turbine wheel.